FREE-FIELD SOUND POWER LEVELS OF OPEN-INLET OPEN-OUTLET FANS AND COMPARISON WITH IN-DUCT MEASUREMENTS

Sound power levels were measured for an axial-flow fan and a centrifug al fan, both having 600-mm-diam impellers. One-third-octave-band sound power levels were measured by microphones at essentially free-field l ocations and in the test ducts for two installation configurations: (1 ) an open inlet and an anechoic outlet duct and (2) an open outlet and an anechoic inlet duct. Two parallelepiped ''rectangular box'' measur ement surfaces and a hemispherical measurement surface were used for f ree-field tests. The requirement in ISO 3744 that the environmental co rrection be not more than 2 dB was not satisfied at some frequencies i n the frequency range of interest, On average, the smallest environmen tal corrections were observed for the smallest measurement surface. Di fferences, as great as 4 dB, between the free-field sound power levels were largest at frequencies for which the environmental correction wa s also large. For plane sound waves, in-duct sound power levels were g reater than the free-field sound power levels. At frequencies where hi gher-order-mode sounds propagated through the test ducts, sound power levels determined by the in-duct method were less than those determine d by the free-field method. This paper discusses reasons for the diffe rences between the sound power levels by the two methods. (C) 1995 Ins titute of Noise Control Engineering.